System for secret multiple-channel carrier intercommunication



Jan. 10, 1939. Si EVY HA 2,143,563

SYSTEM FOR SECRET MULTIPLE-CHANNEL CARRIER INTERCOMMUNICATION FiledMarch 5, 1957 3 Sheets-Sheet l INVENTOR.

Jan. 10, 1939. s. .1; LEVY ET AL CHANNEL CARRIER INTERCOMMUNICATIONSYSTEM FOR SECRET MULTIPLE- 3 Sheets-Sheet 2 Filed March a, 1937 W3 ank99 Jan. 10, 1939. s. J. LEVY ET AL ,1

SYSTEM FOR SECRET MULTIPLE-CHANNEL CARRIER INTERCOMMUNICATION FiledMarch 5, 1937 3 Sheets-Sheet 3 ATTORNEY Patented Jan. 10, 1939 UNITEDSTATES PATENT OFFICE SYSTEM FOR SECRET MULTIPLE-CHANNEL CARRIERINTERCOMMUNICATION Application March 5, 1937, Serial No. 129,140

6 Claims.

The present invention relates to a system of carrier frequency intocommunication over electric light and power lines, or other metallictransmission media, which is especially adaptable to be used in officebuildings, factory buildings, apartment houses and the like. Moreparticularly, the present invention relates to a multi-channel systemwherein communication between different stations is effected by usingdifferent bands of carrier frequencies.

It is the object of the present invention to provide a secretmulti-channel system, wherein any two stations may intercommunicatewithout any other station being able to interfere with suchintercommunication and, in particular, to listen in on suchintercommunication.

It is the further object of this invention to provide a multi-channelsystem, the physical characteristics of which lend themselves to massproduction, whereby the first costs of the system are drasticallyreduced.

It is a still further object of the present invention to provide asystem wherein, if any two stations are intercommunicating and a thirdstation tries to contact either one of the intercom-, municatingstations, no interfering beat tone of the kind hitherto encountered iscreated.

It is a further object of the invention to provide a multi-channel'system in which all frequencies to be modulated are dependent upon thecommon control frequency in a manner such that no independent variationof any of the modulated frequencies may occur.

It is a still further object of this invention to adapt a multi-channelsystem for use in office buildings and the like, in which roomsbelonging to the same group or suite of ofiices are able tointercommunicate without interfering with stations provided in any otherpart of the building.

Other objects, and the manner in which the same are attained, willappear from the following description.

In multi-channel communication systems of the audio frequency typehitherto known, complete secrecy of intercommunication between any twostations without interference from any other station has been obtainedonly by adding a comparatively complicated equipment to thecommunication apparatus, thus considerably increasing the cost thereof.In consequency of these drawbacks, and in particular of the increasedcost or such systems, the lower priced forms of audio frequencycommunication systems were not adapted to insure secrecy ofinterccmmunieaticn between any two stations.

As will be described below in detail, the present invention succeeds inproviding a system insuring complete secrecy for the intercommunieationof any two stations, without thereby complicating the equipment orincreasing, to any substantial extent, the cost thereof.

Furthermore, by providing a multi-channel system of carrier frequencycommunication in which the various substations are identical with thesole exception that they are tuned to different receiving frequencies,the present invention succeeds in providing a system which isparticularly suitable formass production.

Furthermore, at each station, by transmitting on different bands ofcarrier frequencies, while receiving on only one band, the presentinvention succeeds in eliminating the beat tone, previously encountered,if and when two stations try to contact the same third station at thesame time, in particular, if and when a third station tries tocommunicate with either one of two intercom-' municating stations.

Moreover, in a carrier frequency transmissionreception system, it is ofprime importance that the unmodulated frequency component, there beingone for each channel, remain fixed and not vary from time to time as theresult of circuit changes caused by inaccuracies in equipment assembly,circuit conditions, changes in temperature, humidity and the like.

The frequency displacement permissible between adjacent channel carriersis determined by the production of audible beats between the carriers.By suitable adjustment of the circuit, the frequency difference betweencarriers may be audible and yet no audible beat tones are produced inthe receiving equipment. For example, with a fixed 1O kilocycledisplacement between carriers the audio output circuits of the receivingequipment may be so adjusted that the kilocycle beat tone is not audibleand still high quality audio reproduction is obtained. Suppose, however,that due to some circuit inaccuracy or change, there is a slippage inthe carrier frequency of one transmitter so that the frequencydifference between that channel carrier and an adjacent carrier dropsfrom 10 kilocycles to 3 kilocycles, it is obvious that a 3 kilocyclebeat tone will be heard in the receiving equipment. Such a frequencycomponent may not be removed, from the receiver by filtering means aswas the case with 10 kilocycles, as to do so would seriously impair theintelligibility of the received signal. The extreme methods resorted toin space radio broadcast transmitting equipment to obtain frequencystability are indicative of the importance of this subject.

In carrier frequency intercommunication systems where the equipment isrequired to operate under conditions such as met in business offices andby inexpert personnel the questions of size and simplicity of apparatusand cost are of the greatest commercial importance and preclude the use,at each transmitter, of elaborate means such as piezocrystal control tomaintain frequency stability. The present invention succeeds inproviding simple effective and inexpensive means for accomplishing thisend.

Interoflice communicating systems, as the name implies, operate, forexample, between oflices, rooms or buildings or between groups ofofllces, rooms or buildings. There may be, for example, one group ofrooms in a given building or a number of distinct groups'of rooms.Intercommunication may be required between the rooms of one group orbetween those of different groups.

In accordance with one embodiment of the present invention, absence ofaudible beat tone interference between carrier frequencies, due toslippage in frequency, is obtained by generation of one master frequencyand use of harmonics thereof for the different channel carrierfrequencies. For example, 10 kilocycles might be the master frequencyand the tenth, eleventh, twelfth and thirteenth harmonics, or, in otherwords, 100, 110, and kilocycles, are the carrier frequencies for afour-station system. The master frequency is supplied over the powerlines or other metallic transmission media to the various transmittingstations where the required harmonic is developed and used as a carrierfor that station. For example, there may be one master frequencygenerator for one group of offices, or one for a given building,supplying a number of different groups of oflices. Or one masterfrequency generator may even supply any given power line network commonto any number of buildings. The master frequency may be generated by atube oscillator of the high vacuum or gas type or obtained in the wellknown manner of beating two higher frequencies, themselves generated bytube oscillators, or it may be generated by a small, very low poweralternator. If tube generators are employed, they may be provided withpiezocrystal control, there being but one crystal control system for arelatively large group of transmitters.

With the master frequency system as herein described, the frequencydisplacement between the channel carriers is obviously the masterfrequency. As here the carrier for each station transmitter is aharmonic of the low master frequency, it is obvious that no circuitchange at a station transmitter can change the value of this carrier.Only a change in the circuit conditions of the master generator canchange the different station carriers, in which case the latter are allsimultaneously changed in a manner such that the difference betweenadjacent carriers is always equal to the value of the master frequencyor in the case where the carrier frequencies are not successiveharmonics of the master frequency, the difference between adjacentcarriers will always be a multiple of the value of the master frequency.Thus, there can be no independent slippage of the different stationcarrier frequencies and hence no interference between the samemanifested in any individual receiver or elsewhere in the system.

Another embodiment of a multi-channel system according to the inventionprovides for local oscillators in each of the sub-stations, thusdispensing with a common control oscillator. In this case each stationmay generate any of a number of individual frequencies which aremodulated and transmitted over a power line network or other metallictransmission media, the receiver in this case also being adjusted to oneparticular frequency band or range. Thus any station may select fortransmission purposes the particular frequency band to which thereceiver to which it is intended to transmit is adjusted, and in thismanner only that particular receiver will be in intercommunication withthe transmitting station, while all other stations are prevented frominterfering with such intercommunication.

The invention may be applied to all types of carrier frequencycommunication apparatus, and, in particular, to the type using aseparate microphone and loud speaker, as well as to the transceiver typewhere a single instrumentality is used as both microphone and loudspeaker.

In the drawings accompanying the present specification and forming partthereof, several embodiments of our invention are illustrateddiagrammatically by way of example.

In the drawings,

Fig. l is a wiring diagram of one embodiment of a multi-channel systemaccording to the present invention, showing a carrier control frequencyoscillator and three sub-stations provided for transmitting andreceiving on harmonics of the control frequency operating over a commonpower network, the sub-stations including separate microphone and loudspeaker arrangements.

Fig. 2 is a similar wiring diagram in which the substations aretransceiver units, employing a single instrumentality as both microphoneand loud speaker.

Fig. 3 illustrates another embodiment of the invention, showing threestations provided with local oscillators in each station, each stationbeing provided for operating on various fundamentals generated by thelocal oscillator; while Fig. 4 illustrates the arrangement of themultichannel systems according to the invention in an office building.

Referring now to the drawings, and first to Fig. 1, A shows a carriercontrol frequency oscillator feeding a control frequency on a power linenetwork indicated at B, a number of sub-stations I, II and III beingoperatively connected with the same power network.

The carrier control frequency oscillator A includes a local oscillator lwhich generates, for example, 25 kilocycles. This high frequency outputis coupled to the grid circuit of the tube 2 by a coupling capacity 3and the grid tuning circuits 4 and 5. The tube 2 is provided as a classC amplifier, the output of which is coupled to the transformer 6 whichin turn is coupled to the line through capacities I and 8. 9 indicates aconventional rectifier with its soothing reactor H and filter capacitiesl2 and 13. This arrangement as indicated at A as a whole is designed toput sufficient carrier at a frequency of 25 kilocycles for example onthe network indicated at B to operate the harmonic generators, describedbelow.

Referring now to the sub-stations I, II and III shown in Fig. 1, thegrid circuit of the tube 20 includes inductance 2i and capacity 22tuned, e. g., to 25 kilocycles, the coupling to line B being effected at23 and 24. The plate circuit of the tube 20 comprises a number ofcapacities 26, 21 and 28 which, according to the position of a switch8;, which is shown to be of the rotary type but which may be of anyother well known type, for example of the lever type, will tune to anyone of the harmonics of 25 kilocycles. Correspondingly, the grid circuitof the tube 29 and the plate circuit of the tube 3| are provided withcapacities 26', 21' and 28', and 26', 21" and 28" respectively, whichare controlled by switches S: and S3, respectively, which areinterconnected with the switch S1, so as to be operated simultaneouslyand correspondingly. The plate circuit of the tube 29 is coupledcapacitively to the tube 3!, provided as a class C amplifier. This isthen coupled to the line through inductance 32 and capacity 33. Amicrophone 36 works into the tube 35, the output of which is worked intothe tube 36. This tube serves the purpose of modulating the desiredharmonic frequency at 3|. The receiver has for its pick-up circuitcapacity 31 and inductance 38 coupled to circuit 4| and 42 tuned to theincoming frequency.

The tube 43 is a converter tube having the converted frequencycontrolled by inductance 44 and capacity 45. Inductance 46 and capacity41 forming a closed tuned circuit, which is coupled to circuit composedof inductance 48 and capacity 49 as a combination are tuned to theconverted or intermediate frequency, which is supplied to the diode oftube The other elements of the tube 5! form a high MU pentode, theoutput of which is amplified by tube 52, which is connected to thespeaker 53 of the receiver. 54 indicates a conventional half-Waverectifier including the filters 55 and 56.

In the operation of the embodiment of the invention according to Fig. l,a carrier control frequency is put on the line by the oscillator A sothat it is picked up by any of the sub-stations I, II and III. Bysuitably controlling the capacities, 26, 21 and 2B, 26', 21', 28 and26", 21" and 28" any desired harmonic of the control frequency may bedeveloped in a manner such that any sub-station may transmit any of apredetermined number of harmonics of the control frequency.

The receiver of each sub-station is adjusted to one particular harmonicof the control frequency which is different from the harmonicscorrelated with the receivers of any other sub-station.

Accordingly, by suitably selecting the desired harmonic of the commoncontrol frequency, for example, substation I may transmit on thefrequency to which the receiver of the sub-station III is adjusted. Inthis manner only sub-station III will receive the communicationtransmitted by sub-station I, while sub-station II is prevented frominterfering with and, in particular, from listening in on theintercommunication between sub-stations I and III. When intending toanswer to sub-station I, sub-station III will select the harmonic towhich the receiver of sub-station I is adjusted so that it will be heardonly by the sub-station I, any interference on the part of sub-stationII, or any other additional sub-station, being again prevented.

Referring now to Fig. 2, this figure illustrates an arrangement similarto that shown in Fig. l, employing, however, transmitter-receiver units,the principle of which we have disclosed in our S81. No. 81,350, filedMay 23, 1936, Patent No.

2,114,718, dated April 19, 1938. As disclosed in the aforesaidapplication, such units utilize all essential elements for bothtransmission and re-. ception of modulated carrier frequencies, the sameinstrumentality, for example, serving as a microphone for transmissionpurposes and as'a loud speaker for reception purposes.

In the lower left-hand corner of the unit indicated at II-2 in Fig. 2,the 110 volt lines of an electric light network enter the unit throughan on-and-off switch indicated at SI. Across the line and ground, atransformer TR.I is inserted. This transformer is of an iron core typesuitable for passing frequencies above kilocycles for example andcutting off some place above 300' and 400 kilocycles. No 60 cycles willpass through this transformer from the 110 volt lines.

T2 designates a radio frequency amplifier tube, amplifying the entireband above 15 kilocycles.

A filter shown at F inserted in the plate circuit of T2 selectskilocycles for example, rejecting all other frequencies and transfersthe25 kilocycle energy to the grid of tube T3. T3 is so biased thatharmonics of 25 kilocycles are developed in its plate circuit, anyone ofwhich can be selected by the key switches indicated at A2 and B3. InFig. 2, these switches shown to be of the lever type, may, of course, beof any other well known type and in particular of the rotary type.

The RF Voltage produced across this tank circuit of tube T3 then passesto the grid of theone of these harmonics being amplified by the tube T6.The output circuit of T6 is coupled through another iron coretransformer TR2, the primary of which is tuned to the harmonic frequencythat it is wished to speak on and the secondary of this transformerbeing untuned, supplies the loss of a class B or class AB linear poweramplifying tube T7, the output circuit of which is connected to thetransformer TR-3, which is also untuned. The secondary of thistransformer is of a reasonably high impedance, as is the primary ofTR,I, so that these trans formers may be maintained connected across the110 volt lines at all times through the blocking condenser Cl.

In the transmitting position, which is accomplished by switching theswitches S4, S5, S5 and .by its suppressor.

When the switches S4, S5, S6 and S! are turned to the receivingposition, the instrumentality SPK, now acting as a loud speaker, isconnected to the output of the power amplifier tube T5, switch S1biasing the suppressor of the tube T6 with a high negative voltage,thereby dropping the plate current of the tube T1 in a manner such thatthere is no background noise when the unit is in'receive position. Asbefore, the transformer 'I'R-l passes the signal voltage from if anotherstation to the grid of the tube T2, and this frequency is then passedthrough the filter indicated at F to the diode section of the tube T4,rectifies this high frequency and passes it to the audio amplifiersection of T4, where the output is amplified by T and put in the loudspeaker.

'I'heoperation of the embodiment of the inventionaccording to Fig. 2 issubstantially the same as disclosed with respect to Fig. 1. However, aswill appear more clearly from the disclosure in our above mentionedco-pending application, this arrangement involves the advantagesinherent in the transceiver arrangement.

Both the embodiments of the invention according to Figs. 1 and 2 succeedin completely eliminating the beat tone formerly encountered when twostations try to intercommunicate with the same third station. Whileformerly communication between any two stations was greatly or evencompletely disturbed as soon as a third station tried to contact eitherone of the intercommunicating stations, this disadvantage is nowcompletely eliminated.

Moreover, independent slippage of the individual frequencies to bemodulated cannot occur according to the invention, since all modulatedfrequencies are dependent upon the common contral frequency by beingharmonics thereof. Thus, whenever the common control frequency changes,all the frequencies to be modulated will change correspondingly in amanner such that their relation to each other is always maintainedconstant. Since beat tones are produced by changes in the relation ofvarious frequencies to be modulated to each other, no beat tones will beheard in the system according to the invention where such relationshipis maintained constant.

Fig. 3 illustrates a further embodiment of the invention, in which,instead of providing a common control frequency oscillator for allsubstations, a number of identical stations are employed, each of whichcontains its own local oscillator. This embodiment of the invention isillustrated by way of example on a number of transceiver units embodyingthe principles of our above mentioned co-pending application, but wewish it to be understood that the invention can just as well be employedon other carrier frequency communication apparatus such as, for example,shown in Fig. 1.

Figure 3 shows three stations a, b and 0. Each station contains its ownlocal oscillator as illustrated at 60. The inductance GI and 62 andcapacities 63', 63 and 63 determine the three frequencies employed viz;two for transmitting respectively to stations b and c and one forreceiving, to which the oscillator-detector tube 60 is adjusted.Capacities 65, 66 and inductance 64 serve to tune and couple the line tothe desired frequencies. Both sets of capacities are shown to beadjusted by switches which preferably are mechanically operatable fromthe transmitreceive switch. The output of the oscillator is fed throughinductance 64 to the line. The sending frequencies of station b would bethe receiving frequencies of station a and 0, while 'the sendingfrequencies of c would be the receiving frequencies of a and b. When theswitch is in the transmitting position the speaker indicated at 69becomes a microphone, the output of which is amplified by the audioamplifier tubes 61 and 68, the latter serving as a modulator tube in itsaction upon an oscillator tube 60.

In receive position, the vacuum tube 60 becomes a detector by insertinga resistance Rs in its plate circuit which stops it from oscillating.Resistance Rs is thrown in and out of the circuit by means of thetransmit-receive switch, members of which are indicated at the right ofthe Fig. 3. In the transmitting position, the plate voltage, derivedfrom rectifier H is fed through choke coil 12 to connection C andtherethrough to choke coil CH, from thence through the shielded lead Llto the transmit-receive switch, and therethrough, through the shieldedlead L2 to the plate of tube 60. In the receiving position this path isopened at the transmit-receive switch and resistance Rs is placed inseries between the plate of tube 60 and the point of common connectionof choke coil 12 and capacity 13. The output of this detector is thenfed through the audio amplifier 61 and then into the output tube 68,which in turn actuates the speaker which now functions as a loudspeaker. II is a conventional half-wave rectifier with its filter system12, I3 and 14.

A filter indicated at 15 and I6 is inserted in the circuit, the purposeof the capacity 16 being to completely short-out the high frequencygenerated by the half-wave rectifier H.

Choke coil 15 presents a high positive reaqtance to the higher spuriousfrequencies of the power supply, resulting from rectification at H.Capacity 16 forms a low reactance shunting path to these samefrequencies. Inductance 15 and capacity 16 prevent the high spuriousrectification frequencies from being impressed upon the line systemwhile inductance l5 individually prevents capacity 16 from acting as ashort circuit to the carrier signal frequencies, which are sent over orreceived from the line.

The operation of a multi-channel system according to Fig. 3 will beapparent from the foregoing description. Here again the receiver of eachstation preferably is adjusted to one particular receiving frequency,which is different from the receiving frequencies of any other stationin the system. The transmitter of each station is adapted to generate asmany frequencies as there are stations in the system minus one, in amanner such that by suitably selecting the receiving frequency of thestation to which it is intended to talk, each transmitter may contactthe desired station to the exclusion of all other receiving stations.Thus, interference by any third station in regard to anyintercommunicating stations is completely eliminated and completesecrecy of intercommunication is insured.

While the miilti-channel system according to Fig. 3 has beenillustrated, by way of example, on a number of transceiver unitsaccording to applicants above mentioned co -pending application, thissystem may equally well be applied to other types of carrier frequencycommunication apparatus and, in particular, may be applied to apparatushaving a separate microphone and loud speaker arrangement.

Fig. 4 illustrates one position arrangement of the multi-channel systemsaccording to the invention in an oilice building.

In Fig. 4, a carrier control frequency oscillator of any desired type,for example, such as shown at A in Fig. l, is indicated at I00, as beingconnected with a power line network indicated at I01. Branch circuits,there being one indicated for each floor of the building, are shown atI02, I03 and I04. In the branch circuit I02, three amases sub-stationsF1, F2 and F3 are indicatedall of which are supplied through the powerline as harmonics of the control frequency generated in the controlfrequency oscillator 100.; Station Fl would transmit on frequencies F2and F3 and receive on Fl. Similarily station F2 would trans mit onfrequencies FI and Hand receive on F2,

etc.

Inthe branch circuit I03, fourstations, G, H, I and K, are shown whichdo not pick up the control frequency, but which are all provided withlocal oscillators and which operate all on the same frequency.

In the branch circuit I04, a two-station arrangement is shown whichincludes the stations F4 and F4. Bothof these stations operate on oneand the same harmonic, e. g., F4, of the control frequency generated bythe control frequency oscillator I00, since there are only two stationsin this case and thus no interference on the part of a third stationwithin this set-up is to be excluded. These stations transmit andreceive on the same harmonic frequency which, of course, must bedifferent from any other harmonic frequencies employed in connectionwith the control frequency oscillator I00.

We wish it to be understood that wherever we use the term loudspeaker inthe present application, we intend to include any well known equivalentof receiving instrumentality as well, such as, for example, telephonereceivers, piezocrystal or any other equivalent thereof.

Numerous advantages are obtained with the secret multiple channelcarrier frequency intercommunication systems according to the presentinvention.

A multi-channel system is created which will provide perfect secrecy ofintercommunication so that interference of any third party with themtercommunication of any two stations 1s completely eliminated. Thenovel system facilitates, and reduces the costs of, the manufacture ofcarrier frequency apparatus by lending itself particularly well to massproduction. The trouble previously encountered when a third stationtried to contact one of two intercommunicating stations, whichmanifested itself in heat tones which frequently made intercommunicationaltogether impossible, is eliminated according to the present invention.Individual slippage of any of the frequencies operated on by anysub-station is avoided due to operation on harmonics of one and the samecontrol frequency. Finally, the systems are particularly adapted for useon various floors and in connection with various groups of oflices inoffice buildings and the like. Thus, the systems are able to fill asteadily increasing demand for reliable and inexpensive ofiiceintercommunication systems.

We wish it to be understood that we do not desire to be limited to theexact details of construction and design shown and described, forobvious modifications within the scope of the appended claims may occurto persons skilledin the art.

We claim:

1. A secret multiple-channel carrier frequency intercommunication systemcomprising in combination a metallic transmission medium such as anelectric network, a source of unmodulated carrier control frequencyenergy supplying said transmission medium, a plurality of distinctgroups of carrier frequency apparatus, each group comprising at leasttwo carrier frequency transmitting and receiving units having theiroutputs and inputs connected with said transmission medium, and means insaid transmitting and receiving units for effecting operation onharmonics of said carrier control frequency, the units of each groupbeing adaptedto operate on one distinct harmonic of the carrier controlfrequency which is different from the harmonics operated on by, anyother group.

2.; A secret multiple-channel carrier frequency intercommunicationsystem comprising in combination a metallic transmission medium such asan electric network, a source of unmodulated carrier control frequencyenergy supplying said transmission medium, a plurality of carrierfrequency transmitting and receiving units having their outputs andinputs connected with said transmission medium and means provided insaid transmitting and receiving units for effecting transmission on anyof a plurality of harmonics of said carrier control frequency, and foreffecting reception on only one distinct harmonic of said carriercontrol frequency, each unit being arranged for reception on oneharmonic which is different from any harmonics correlated with any otherunits.

3. A secret multiple-channel carrier frequency intercommunication systemcomprising in com bination an electric wire network of a restrictedtype, a source of unmodulated carrier control frequency energy supplyingsaid network, a plurality of distinct groups of carrier frequencyapparatus, each group comprising at least two carrier frequencytransmitting and receiving units having their outputs and inputsconnected with said wire network, and means in said transmitting andreceiving units for effecting operation on harmonics of said carriercontrol frequency, the units of at least one group being adapted tooperate on one distinct harmonic of the carrier control frequency whichis different from theharmonics operated on by any other group, and atleast one other group including a larger number of units, each of saidlast-named units being adapted to transmit on any of a plurality ofdistinct harmonics which are different from the harmonics operated on byany other group, and to receive on only one of said plurality ofdistinct harmonics of said carrier control frequency.

4. The method of carrier frequency intercommunication on metallictransmission media such as an electric network, comprising incombination the steps of supplying a carrier control frequency to saidnetwork, coupling to said network a plurality of groups of carrierfrequency transmitting and receiving apparatus, each group comprising atleast two transmitting and receiving units, and operating all the unitsbelonging to one particular group on a distinct harmonic frequency ofsaid control frequency, while operating the other groups of units eachon a different harmonic frequencies of said control frequency.

5. The method of carrier frequency intercommunication on an electriclight and power network, comprising in combination the steps ofsupplying a master control carrier frequency to said network, coupling aplurality of carrier frequency transmitting and receiving units to saidnetwork, transmitting from each unit on any of a plurality of harmonicsof said master control frequency, while receiving in each unit on onedistinct harmonic, each of said plurality of harmonies thus beingcorrelated for reception purposes with one particular unit.

6. The method of carrier frequency intercommunication on an electriclight and power network, comprising in combination the steps ofsupplying a carrier control frequency to said network, coupling to saidnetwork a plurality of 5 groups of carrier frequency transmitting andreceiving apparatus, each group comprising at least two transmitting andreceiving units, operating the units belonging to at least oneparticuiar group of a distinct harmonic frequency of said controlfrequency, and operating the units of at least one other group on aplurality of harmonics different from those operated on by any othergroup, in such a manner that each 01 said units transmits on any of saidplurality of harmonics, and receives on only one of said plurality ofharmonics.

SOL J. LEVY.

JOSEPH LAWRENCE CASSELL.

